1. Field
The present invention relates to the field of communications. In particular, this invention relates to a system and method for enhancing the quality of audio transferred over a network.
2. General Background
For many years, networks have been used to transfer voice data between a source and a destination. As shown in FIG. 1, a typical voice-based network 100 comprises a local (near-end) telephone 110 and a remote (far-end) telephone 120 coupled together over a bidirectional transmission path featuring a two-wire xe2x80x9clocal loopxe2x80x9d line 130 in communication with a long-distance (four-wire) xe2x80x9ctrunkxe2x80x9d line 131 of network 100. The trunk line 131 includes a pair of separated two-wire lines to transmit information to and receive information from remote telephone 120.
In particular, before reaching local telephone 110, an incoming voice data is provided to a hybrid circuit 140 in a local terminal 150. A xe2x80x9clocal terminalxe2x80x9d is equipment maintained by a telephone company to route voice data to a telephone in a predetermined area. Hybrid circuit 140 is a transformer circuit that enables the voice data to be transmitted over different types of transmission lines. For example, hybrid circuit 140 may be designed to act as an interface between local loop line 130 (e.g., twisted pair) and trunk line 131 (e.g., fiber optics, microwave or any digital transport medium). Since hybrid circuit 140 is not perfectly terminated, a talker echo will occur.
xe2x80x9cTalker echoxe2x80x9d is generally defined as a particular amount of energy associated with incoming voice data that is reflected back to the caller from which the voice data originated. Typically, for voice-based networks, talker echo is experienced when a person speaking on the telephone can hear his or her own spoken words after a short delay. To mitigate talker echo, an echo canceler 160 is coupled to input and output transmission lines of trunk line 131.
As further shown in FIG. 1, echo canceler 160 is a circuit that observes, but does not modify, an incoming voice data before transmission to local telephone 110. Instead, echo canceler 160 performs an echo cancellation technique which includes the following: (1) estimating an amount of talker echo returning to the caller, (2) simulating the estimated echo, and (3) subtracting the estimate echo from outgoing voice data. Then, in most cases, a nonlinear processor (NLP) 161 of echo canceler 160 performs a center clipping transfer function to remove any echo residue as shown in FIG. 2. The removal of the echo residue is accomplished by clipping a portion of the outgoing voice data if it falls below selected clipping amplitude threshold levels A1 205 and -A1 210. These levels are measured in decibels such as decibel power levels (dBm0) for example. The clipped portion of the outgoing voice data is substituted for comfort noise that emulates a background noise level in order to avoid perceivable, audible artifacts.
One problem with the above-described echo cancellation technique is that the conventional NLP analyzes numerous parameters to determine the clipping amplitude threshold. In many cases, these parameters vary substantially over a short time duration that, in some situations, causes clipping errors to occur. For example, presume that NLP uses the amplitude level of far-end incoming voice data to calculate (i) the clipping amplitude threshold and (ii) whether a double talk condition exists (e.g., the two callers are talking simultaneously). If both callers are talking at substantially different amplitude ranges (e.g., the voice level of the far-end caller is approximately 10 or more decibels xe2x80x9cdBsxe2x80x9d higher than the voice level of the near-end caller), the outgoing voice data may be accidentally clipped.
Hence, it would be desirable to provide a system and method that minimizes the complexity of the NLP and enhances the quality of all audio by reducing the likelihood of erroneous clipping of the audio. This is accomplished through dynamic adjustment of the clipping amplitude threshold.
In one embodiment, the present invention relates to an echo canceler comprising a nonlinear processor utilizing a dynamic clipping amplitude threshold to determine whether or not to clip outgoing digital signals. The dynamic clipping amplitude threshold is increased when the ratio between the residue echo and echo is decreased over a prescribed period of time. In one embodiment, the residue echo is taken as a difference between a true echo and an estimated echo initially emulated by the echo canceler. This mitigates the tendency of erroneous clipping of the digital signals. Likewise, when the ratio is increased over the prescribed period of time, the dynamic clipping amplitude threshold is decreased.